Components formed of specialty materials such as superalloys are used in a wide variety of industrial applications under extreme operating conditions. In the energy generation field, working components invariably must be coated to increase their resistance to surface degradation such as oxidation, corrosion, erosion and wear over extended time periods. For example, gas turbine components exposed to temperatures over 1500° F. such as shrouds and airfoils typically have been coated during their original manufacture and/or during downtime repairs to increase the protection against oxidation, corrosion and particle erosion when exposed to an oxidizing atmosphere for long periods at high temperatures.
In the past, conventional protective coatings have been applied to metal substrates using techniques designed to optimize the microstructure and mechanical properties of the coating. However, the coatings tend to be expensive, involve complicated process controls and consume a considerable amount of downtime to coat the article. Examples of such processes include low pressure plasma spray (LPPS), vacuum plasma spray (VPS), high velocity oxygen fuel (HVOF), air plasma spray (APS), and electron beam physical vapor deposition (EEPVD). Turbine components have also been repaired using diffusion aluminides applied in vapor, pack or slurry processes. Unfortunately, many known prior art coatings tend to become brittle over time or crack due to thermal cycling and metal fatigue occurring when the turbine engine is taken in and out of service. Modifications of the coatings to make them less brittle over time often result in a lower resistance to oxidation.
The concern over wear and oxidation resistance of gas turbine components is particularly acute for metal structures formed from superalloys such as those used in multi-stage engines operating at elevated temperatures, e.g., at or above 1000° C. Without a protective coating on the exposed metal components, the oxidizing atmosphere of a gas working fluid at high temperatures can rapidly change the chemistry, and thus the properties of the metal substrate. A significant debit in material properties in one area can be very detrimental to the mechanical integrity and reliability of an entire system. Thus, various methods to prolong the life of components have been developed to cover critical component surfaces with protective coatings. Although the presence of aluminum in protective coatings improves oxidation resistance, excessive aluminum can also decrease the coating ductility, resulting in cracking during prolonged service and eventual loss of the initial benefits of the coating.
Most oxidation-resistant coatings used with superalloys comprise alloys having the general formula MCrAlY, where M includes iron, nickel, and/or cobalt. Preferably, the coatings are applied as the final layer with a smooth, uniform and controlled thickness in order to achieve maximum life and aerodynamic efficiency. Conventional thermal spray techniques that have been used to apply such coatings have positive and negative attributes, depending on the operating environment, component size, and nature of the working fluid. VPS applications, for example, are useful when the final protective coating must be free of metal oxides.
VPS and HVOF techniques, on the other hand, are less effective in applying coatings to regions of a substrate that are inaccessible due to physical limitations of the spray equipment which may be too large or cumbersome to use in small areas to have a line of sight gun angle for reasonable deposition rates and an acceptable microstructure. Most thermal spray processes also include one or more masking steps that can be costly and time-consuming in carrying out localized repairs. Other known coating systems likewise tend to be expensive, require complex process controls and take considerable down time to effectively and reliably coat target components.